1. Field of the Invention
The present invention relates to a secondary battery, in particular, a redox flow secondary battery (also called "a redux battery"), more specifically, an electrode structure of a high power redox battery, usable at high current density, and having low internal electrical resistance and pump power loss.
2. Description of the Related Art
In recent years, global warming due to an increase in atmospheric carbon dioxide is being focussed on as a problem resulting from the large-scale consumption of fossil fuels. Considering the circumstances, the development of solar batteries and clean sources of energy, has been actively pursued- However, since solar batteries cannot generate power at night or on rainy days, the development of suitable secondary batteries has been required.
Further, the efficiency of conventional power plants is decreasing due to the great difference in power demands between day and night. Therefore, leveling out the operation of the power plants by large scale power storage batteries has great significance for saving energy. Although storage of electric power has been a long-cherished dream for people in the field, only pumped storage power plants have been realized at the present stage. Accordingly, large scale power storage batteries are still a real necessity.
Redox batteries have great possibility as a new form of secondary batteries, because the batteries are chargeable by adapting the output voltage of solar batteries with a tap changer, and are easily scaled up due to their simple structures.
In redox batteries, charging and discharging are carried out by oxidation-reduction in which liquid active materials in the positive electrode and the negative electrode are circulated to liquid permeable electrolytic vessels. The redox batteries have the following advantages compared with conventional secondary batteries:
(1) Since storage capacity increases with increased container volume and active material contents, scaling-up the electrolytic vessel is not required unless the output power is increased. PA0 (2) Since the positive and negative electrode active materials can be completely preserved separately in the container, the possibility of self discharge is low, differing from batteries in which active materials contact with electrodes. PA0 (3) The reaction in the batteries is simple, because the charge and discharge reaction of the ions of active materials (electrode reaction) merely exchanges electrons on the electrode surfaces, differing from zinc-boron batteries in which zinc ions precipitate on the electrodes. PA0 (1) the septum side layer of said liquid permeable electrode is a porous electrode comprising carbon fiber having a diameter of 2 to 20 .mu.m, with a surface area is 3 m.sup.2 /g or more; PA0 (2) the bipolar plate side layer of said liquid permeable electrode comprising a carbon fiber having a diameter of 2 to 20 .mu.m, and a surface area is 2 m.sup.2 /g or less.
Although iron-chromium batteries, as a type of redox flow secondary battery, are known, the batteries have such problems as low energy density and mixing of the iron with chromium through the ion exchange membrane. Thus, a vanadium battery was proposed in Japanese Patent Laid-Open No. 62-186473. Vanadium batteries have such excellent properties as high electromotive force and battery capacity. Further, the electrolytic solution consists of one metal component. Therefore, even if positive and negative electrode electrolytic solutions mix together through the septum, these solutions can be easily regenerated by charging. However, in these conventional redox batteries, since available current density is around 60 mA/cm.sup.2 at most, they are impossible to use at higher current densities.
In order to maintain higher power efficiency by adapting a higher current density of more than 80 mA/cm.sup.2, in particular, 100 mA/cm.sup.2, the cell resistance should be 1.5 .OMEGA..multidot.cm.sup.2 or less, preferably 1.0 .OMEGA..multidot.cm.sup.2. Considering the resistance of the electrodes and the conductivity of the electrolytic solutions as well as cell resistance, the cell thickness of the positive electrode chamber and the negative electrode chamber must be reduced. However, reduced cell thickness necessitates an increase in pump power for electrolytic solution permeation, resulting in decreased energy efficiency as shown in the following equation (1). Therefore, novel liquid permeable porous electrodes, which can reduce the internal resistance of a battery cell without reducing cell thickness, or which can improve the permeability of the electrolytic solution without increasing the internal resistance of the cell, must be developed. ##EQU1##
The differential pressure .DELTA.P, when the electrolytic solution passes through the liquid permeable porous electrode, can be expressed as the following equation (2) (Refer to "Kagaku Kogaku Enshu (Chemical Technology Exercise)" by Mitsutake, Sangyo Tosho, 1970). ##EQU2## Wherein, .mu. represents the viscosity of the liquid, u represents the speed of the liquid passing through the electrode, s represents the specific surface area per unit weight, L represents the thickness of the electrode, and .epsilon. represents the rate of porosity, respectively.
It can be seen from equation (2) that effective methods for reducing the differential pressure .DELTA.P are either increasing the rate of porosity .epsilon. of the electrode or decreasing the specific surface area s of the electrode. However, when increasing the rate of porosity .epsilon., the electrode area per unit volume decreases so that reactivity decreases and the internal resistance undesirably increases. Further, decreasing the surface area of the electrode also causes a decrease in electrode area per unit volume, resulting in an undesirable decrease in reductive activity. Thus, equation (2) demonstrates that reducing internal resistance is incompatible with reducing differential pressure .DELTA.P.
Conventional methods of reducing internal resistance are, for example, reducing cell thickness by excessively pressing the liquid permeable porous electrode, or increasing the total number of activation sites for oxidation-reduction per unit volume by densely packing the carbon fiber of the reactive, liquid permeable porous electrode. However, both methods decrease the rate of porosity, resulting in an increased differential pressure .DELTA.P.
Therefore, even though charge and discharge efficiency increases, these methods decrease total energy efficiency due to high pump power loss.
In contrast, a method for reducing pump power loss has been proposed by forming grooves, for circulating the liquid, along the flowing direction of the electrolytic solution on the carbon plate collector in Japanese Patent Laid-Open No. 2-148659, and another by placing a highly circulative, porous insulator between the porous electrode and the septum in Japanese Patent Laid-Open No. 2-148658. However, because cell resistance described in both of these pieces of prior art is only around 1.8 .OMEGA..multidot.cm.sup.2, they are not suitable for use in redox batteries of high current density.
As described above, it is essential for high current density to provide a large quantity of electrolytic solution. However, conventional electrodes have a structure wherein either cell thickness is reduced so as to decrease internal resistance, or circulation groove size is reduced by increasing the electrode density so as to decrease the electrode resistance, so that such problems as increased pressure loss of circulation in the electrodes and significantly increased pump power loss occur.